Putting time’s mysteries in order

A tick, a tock, a swiftly shifting digit. We have many ways of keeping track of time. We parse it into years, months, days, hours, minutes, seconds. We mark its movement obsessively, plan our days around it, use its form to bring meaning to stories with a beginning, middle and end (chronologically is one of our most common ways to organize information).

Cells use time’s rhythms to organize different functions. Precision timing of neurons and muscles, directed by the brain, controls the body’s movements. Time is a tool that brings order to our lives. It is ironic, then, to discover that physicists explain time as a product of disorder: Its forward direction reflects the unalterable tendency to increasing messiness in the universe.

Investigating both the orderly and disorderly dimensions of time provides the focus for this special issue, aimed at offering new perspectives on something most of us take for granted. For instance, Andrew Grant explores a fresh idea to explain the one-way direction of time’s flow, a conundrum that has perplexed physicists for over a century. The new proposal suggests that the universe actually does run backward as well as forward in time (as our brains would perceive it).

As it turns out, our brains’ perception of time is quite complex, as Laura Sanders reports. She describes the presence of multiple timekeepers in the brain and explores some of the troubles scientists face in studying how the mind pieces together data from its many timepieces.

Tina Hesman Saey’s feature on biological clocks takes an evolutionary perspective, updating readers on new efforts to explain how and why circadian rhythms might have evolved in living things to begin with. Minimizing exposure to threats in the environment such as ultraviolet light and oxygen may have fueled the development of internal clocks. So too might have the cell’s search for efficiency — saving energy by doing things in bursts, instead of continuously.

As our timeline of timepieces illustrates, the human ability to track time grows ever more precise, as does our understanding of how time’s nature influences our world, brains and bodies. But many mysteries endure. And probably will for a long time.